Servo power amplifier



1963 v H. R. PHOTINOS 3,101,437

' SERVO POWER AMPLIFIER Filed July 1, 1960 IIO EXCITATION VOLTAGE QM W United States 3,101,437 I snnvo rowan LIFIER Harry R. Photinos, .San Diego, Calit assignoito The Ryan Aeronautical Co., San Diego, Calif. 1

7 Filed .luiy 1, 1960, Ser. No. 40,299

1 Claim. ($1. 318-207)' The presentqinvention relates generally to amplifiers and more particularly to a servo power amplifier.

The primary object of this invention is to provide a servo power amplifier for driving a twophase servo motor, the amplifier beingfullytransistorized and having an inherent 90 degree phase shift which isrequired tfior such a motor.

Another object of this invention is to provide a servo power amplifier in which the, 90 degree phase shift is 3,101,437 Patented Aug. 210, 1963 power supply and prevent any ripple from appearobtained ras'a feature of the basic amplifier circuit with out the useof a series capacitor type phase shifter which would requireline frequency regulation for proper servo operation. 7

Another object of close approximation for normal line frequency variations and is independent of temperature and aging.

Still another object of this invention is to provide a this 1 invention is jto provide a servo A power amplifier in which the phase shift is 90 degrees to servo power amplifier having an emitter follower input stage with high input impedance and a push pull output stage, coupled in such a manner. that no output transformer is needed for coupling and impedance matching of the load.

A further object of this invention is to provide aservo i power amplifier which contains a minimum of components and has a high degree of stability over a wide range of operating conditions. I

Finally, it is an ohjectto provide a servo power am- I plifier of the aforementioned character which is simple and convenient to build and install and v v which will give generally efficient and durable service. A

With these and other objects definitely in view, this invention consists in the novel construction, combination and arrangement of elements and portions, as will he hereinafter fully described'in the specification, particularly. pointed out in the claims, and illustrated in the drawing which forms a material part of this disclosure, and in which the single figure is a schematicwiring diagram of the amplifier.

' ing in the input.

The emitter 16'isconnected directly to the base22 and vallso to the common conductor 58 throughan emitter v load resistor 76, which optimizes operating current of the input trans'ist'orltl, thecollector 14 being connected to common conductor 72. The emitter 26 is connected to common conductor 58 through a gain adjustment potentiometer 78 and also through an emitter hias resistor 80 and emitter bypass capacitor; 82 in parallel. For specific gain requirements the potentiometer 78 can be replaced by a fixed resistorifdesired. Collector 24 is coupled to one end of the input winding .84 of driver transformer 28,

the other end of said winding being connected to common conductor 72. The ends ofthe output winding86 of output transformer 28 are connected. to the bases 32 and 42 of output transistors 30 and 40, respectively, said out- 1 put winding having a center tap88 which is coupled to common conductor 72 through a voltage dropping resistor Q0 to control output stage hias. Thecenter tapn88 is alsooonnected through a voltage divider resistor 92 to common conductor SSQsaid resistor acting as a shunt in the output stage bias network, and coupledin parallel with resistor 92 i s-[a negative temperature compensation resistor 94. The emitters 36 and 46 are connected to common condulctor 58 through bias stabilization resistors 96 and 98 and collectors 34 and 44 are connected to output termi "nals 100 and 1 02, respectively. Coupled across terminals 100 and 102 is a tuning capacitor 104 to optimize amplifier load condition, the value of said capacitor being de;

pendent on motor load and frequency. In instances where coupling exists between output and input signals, the output terminals 100 and 102 must be grounded through capacitors 1% and 108, respectively. I j

Miniature servo motors, tor use withwhicih the present amplifier is intended, are of the two phase variety requir- A ing a 90 degree phase shift between the winding excitation voltages, this type of motor eliminating the need The amplifier includes basically an input stageutilizing an emitter follower input transistor 10 having a base 12, collector 14 and emitter 16, and a grounded emit ter driver transistor 2o having a base 22, collector 24 and I emitter 26, which is coupled through a driver transformer to a grounded common conductor 58 and coupled across the input, from terminal 52 to the input end of resistor 54 are two clamping diodes and 62 connected in parallel to conduct in opposite directions. The base 12i's connected through 'a voltage divider resistor '64 to the common conductor 58 which leads to the negative 1 side of a DC. power supply, indicated as a battery 66. The

. base 12 is also connected through a voltage divider re for an output transformer. The servo motor 110 has an input winding 112 With a center tap 114 and anexcitation winding 116 connected to a suitable source of excitation voltage. Output terminals 100 and 102 are connected to opposite ends of the input winding 112, the center tap 114 being connected'to a center output terminal 118 which is coupled to the positive side of battery "66. Bh'aseyangle accuracy is very important in small servo motors, since torqueisjproportional to the cosine of angular error, or diiference from the desired phase angle, at the motor windings, theerror being accumulated from all sources.

A In the circuit illustrated, the phase shift network comprises a constant current collector driving source, which is the driver transistor 20, coupled to the primary or input winding84 of driver transformer 28, this providing a 90 degree leading phase shift. The

particular method of couplin-gthe driver transistor to its driver transformer avoids the need for a tuning network for resonance tuning of the load and provides the inherent 90 degree phase shift necessary for operation of a two phase selvo motor. The driver stage is direct coupled to the input stage'and drives a class B biased, pushpull output stage.

Inthe input stage, the forward conduction characteristics out the clamping diodes 60 Iand62 protect the input sister 68 and a filter resistor to a further common conductor 72, which is connected to the positive side of battery 66. The junction of resistors 68 and 70 is coupled to ground through a filter capacitor 74, said capaci tor and the resistor 70 actingto filter out ripple-of the transistor 10 from amplitude overloading and limit input voltage to the required degree, without imposing a penalty in gain or input impedance for operation below the critical level. The direct coupling of driver transistor 20 to input transistor 10 provides good current regulation for both stages, the transistor current regulation factor Ibeingvery high, even when using transistors of low beta value. Addition of a negative temperature commange;

p'ensat'ion resistor 94 to the voltage dividing resistors 90 and 92 ensures maximum gain control, consistent with transistor current regulation, over a wide temperature Bias stabilization resistors 96 and 98 control emitter degeneration and ensure against thermal runaway of output transistors having high collector to base leakage current. While various types of transistors may be used in the output stage, silicon power transistors (are desirable for their stable current gain characteristics and ease'of temperature compensationf Due to the absence of separate motor phase shift and driver tnansfiormer tuning networks, thelamplifier does not require negative loop feedback Iforamplifier phase stabiliznation. great advantage in the miniaturizedequipment used in airborne electronic apparatus, the inherent reliability and minimum need for adjustmentv or servicing being very desirable.

'A non-feedback servo power amplifier, comprising:

a transistor connected a constant-current commone'mitter configuration, whereby said transistor has a relativelythigh elfect-ive resistance;

The extreme simplicity of the circuit is 21 means for applying an input signal to said transistor;

a transformer having a low inductance primary winding and a secondary winding;

means fior applying the output from said transistor to said primary winding, whereby the signals across said transistor and said primary have a stalbie substantiaily 90 degree phase difference as the ratio of; said resistance to the inductive reactance increases, said stability being inherent and not requiring negative feedback; at push-pull amplifier; I g means tor causing said secondary to drive said pushpull iamplifier; 1 a servo motor having l3.

tapped second winding; g

. means for applying a first driving signal to said first winding of saidmotor, said first driving signal being in phase with said input signal;

first winding and a centermeans for applying a second driving signal to said sec- 0nd winding of said motor, said second driving signai being obtained from said push-pull amplifier, whereby the second driving signal is substantially 90 degrees out of phase with said first driving signal,

and may cause said motor to rotate in either. direction in accordance with said input signal.

References Cited in the file of this patent UNITED STATES PATENTS,

Rickner et a1. Sept. 6, 1960 OTHER REFERENCES Darrv Transistor and Hybrid Auto Radios, Radio Electronics, December 1957, pages 42 and'43.

Yaeger July 22, 1958 r 

